1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly to an optical substrate chip carrier.
2. Description of the Related Art
As noted in Wikipedia, common commercial circuit packaging includes the dual in-line package (DIP), pin grid array (PGA), ball grid array (BGA), leadless chip carrier (LCC) packages, and surface mount package with leads formed as either a gull-wing or J-lead. A surface mount package typically occupies an area about 30-50% less than an equivalent DIP, with a typical thickness that is 70% less. This package has “gull wing” leads protruding from the two long sides and a lead spacing of 0.050 inches. In a Flip-chip Ball Grid Array (FCBGA) package the die is mounted upside-down (flipped) and connects to the package balls via a package substrate that is similar to a printed-circuit board rather than by wires. When multiple dies are combined on a small substrate, its called an MCM, or Multi-Chip Module. A big MCM may be considered to be a small printed circuit board (PCB).
Individual components or surface mount devices (SMDs) may be placed on a die or printed circuit board (PCB) using a SMT (surface mount technology) component placement system, commonly called a pick-and-place machine. Generally, these robotic machines are used to place surface-mount devices (SMDs) onto a PCB. They are used for high speed, relatively high precision placing of broad range of electronic components, like capacitors, resistors, integrated circuits onto PCBs. These systems normally use pneumatic suction nozzles, attached to a plotter-like device to allow the nozzle head to be accurately manipulated in three dimensions. Additionally, each nozzle can be rotated independently.
Wire bonding is the primary method of making interconnections between the leads of an IC package and the PCB during semiconductor device fabrication. Wire bonding can also be used to connect one PCB to another. Wire bonding is generally considered the most cost-effective and flexible interconnect technology, and is used to assemble the vast majority of semiconductor packages. Bond wires usually consist of one of the following materials: aluminum, copper, or gold. Wire diameters start at 15 μm and can be up to several hundred micrometers for high-powered applications. Junction size, bond strength, and conductivity requirements typically determine the most suitable wire size and material for a specific wire bonding application.
An optical component such as a laser diode or photodiode converts signals between the optical and electrical domains. Driver circuitry such as transimpedance amplifiers (TIAs) is typically used to convert electrical current generated by a photodiode into a (larger) voltage for subsequent processing. Likewise, the electrical interface of a laser diode is typically connected to laser driver circuitry to supply adequate current (i.e. voltage-to-current conversion).
There have been problems is scaling up the design of optical circuitry into an MCM that includes a plurality of optical components. The signals to-and-from the optical components and the driver circuitry operate at a high frequency, on the order of a 10 gigahertz bandwidth. At these frequencies the length of the wire bonds between the optical components and driver circuitry becomes critical. However, as more components are added to an IC, it becomes increasing difficult to locate a driver circuit directly adjacent to its corresponding optical component to minimize lead lengths. Likewise, even if the drivers can be located adjacent the optical components, the wirebonds from the drivers to the IC package leads must also be considered. Since the signals on these wirebonds operate at the same high frequencies, these lengths must also be short as short as possible. Ideally, these driver-to-package lead signals path lengths could be minimized if the electrical signals could be conducted from the drivers via a FCBGA interface, however this would require that the optical components be located on the same die as the drivers. Otherwise, the operation of soldering the FCBGA interface to a motherboard would likely cause the reflow of any solder contacts, such as wirebonds between the optical components and the drivers. Current technology is unable to practically fabricate optical components and drivers on the same die.
In an MCM with a plurality of optical components, it would be advantageous if the length of wirebonds between the optical driver circuit and MCM package leads could be minimized.
It would be advantageous if the driver-to-package lead wire bond lengths could be minimized by eliminating the wire bond connections between the optical components and drivers.